Checking transmission

Hi dears,
One car mechanic, while in transmission in D, no pressing accelartor (ideling) and car moving forward down the slope, suddenly shifted to reverse. The engine did not stall but vehical stopped without brake on slope. He told the vehicle should not move or move according to the shift against the slope. If it is not like this, then transmission is damaged. If it is true I want to know at what RPM and how much slope that the vehicle should not move along the slope. Thanks.
Azharj